Engineers are constantly looking for ways to improve the efficiency and performance of two cycle engines. Several conflicting demands on some engines have placed undesirable spacial limitations relating to the intake or exhaust valve(s) as well as the incorporation of a suitable fuel injection system. In the case of two cycle engines, an ideal scavenging configuration provides for "through flow" or "uni-flow" by the addition of exhaust or inlet valves in the head. However, the addition of the valve train in today's diesel two cycle engines causes two problems: (1) increased manufacture and maintenance costs; and (2) a compromise between the valve location for breathing and optimal location of the injector for combustion.
In addition to the problems identified above, two stroke diesel type free piston engines have particular limitations that are in need of improvement. In general, the power density of a free piston engine can be increased by reducing engine size two ways: (1) a shorter stroke with a proportionally increased frequency; and (2) a reduced piston diameter with increased frequency (accompanied by an increased mean piston speed). The primary limitation to the latter is intake air flow, or scavenging. The power density limitations of the free piston engine could be significantly overcome by incorporating uni-flow scavenging advantages in order to allow for higher mean piston speeds.
In many engines, both the gas exchange valve(s) and the fuel injection system are coupled in their operation to the piston position within the engine. Engineers have recognized that combustion efficiency and overall engine performance can be improved by de-coupling the operation of the fuel injection system from the position of the piston in the engine. In this regard, Caterpillar Inc. of Peoria, Illinois has seen considerable success by incorporating hydraulically-actuated electronically-controlled fuel injectors into engines. These fuel injection systems allow an engine computer to inject a calculated amount of fuel, often in a pre-determined way, into the combustion space in a timing that is based upon sensed operating conditions and other parameters.
In part because of the gains observed by the incorporation of hydraulically-actuated electronically-controlled fuel injectors, engineers believe that further improvements in performance and efficiency can be gained by also de-coupling at least one of the gas exchange valves from the piston position in a two cycle engine. In other words, it is also desirable that at least one of the exhaust or intake valves be electronically-controlled in order to control exhaust and intake portions of the engine cycle in a more independent and efficient manner for a given operating condition.
The present invention is directed to overcoming one or more of the above and other problems, as well as improving the efficiency and performance of two cycle engines in general.